Next tonight to those food trucks growing more and more popular, one of them bursting into flames in Philadelphia. We've just learned the cause. Authorities have a warning tonight for all of us as millions prepare fo grill on the 4th. ABC's linsey Davis tonight. Reporter: Keep your eyes on this food truck. Watch, without warning, as it explodes into a massive fireball. Look again. Flames span the street in both directions, injuring 13 people, among them, the mother and daughter inside that truck. Chaos. I came around the corner and saw flames coming out of the truck, the woman on the ground. Reporter: Police think a propane tank on the truck was leaking. It's invisible vapor ignited by the hot grills inside. A propane leak was also blamed for this blast during a game at Fresno Christian high school. But it's not just food trucks. Similar propane tanks will fuel many backyard barbecues. In 2012 gas grills sent an estimated 4,000 people to the hospital. Our colleague, ESPN anchor Hannah storm was one of them. A huge fire ball exploded. Reporter: She relit her grill, igniting an invisible propane cloud. Propane is a monster and you have to respect it. Reporter: If your grill goes out, turn everything off and wait 15 minutes before relating. Always keep the lid all the way open when lighting the grill. Watch what can happen when gas builds up. Try rubbing dish soap on those hoses and connections. Leaking gas will cause it to bubble up. Just a few ways to stay up now that grilling season is heating up.

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